The most widely used method for synthesizing an azomethine or indoaniline series dye consists of an oxidative condensation reaction between an active methylene- or methine-containing compound and a p-phenylenediamine in the presence of a base and an oxidizing agent. However, the dye product is present together with the oxidizing agent and the base reagents used if the reaction is carried out in a one-phase system. Therefore, the method suffers from disadvantages in that the dye product decomposes to yield undesirable by-products, a good deal of solvent is required in producing a water-insoluble dye, and so on.
In an effect to overcome these disadvantages, a method has been proposed in which the reaction is conducted in an ethyl acetate-water two-phase system, as disclosed, for example, in JP-A-59-182839 (The term "JP-A" as used herein means an unexamined published Japanese patent application), JP-A-01-110985. According to this method, the produced dyes are extracted with ethyl acetate, and, therefore, separated from both the oxidizing agent and base which are present in the aqueous phase, so the decomposition of the produced dyes is reduced. In addition, the amount of solvent can be reduced in quantity as compared with one-phase systems. However, azomethine or indoaniline series dyes which have been used in particular for thermal transfer processes possess insufficient solubility in ethyl acetate. Consequently, it is still necessary to use ethyl acetate in a large quantity, though the quantity is smaller than that in one-phase systems, resulting in a considerable lowering of productivity and a sharp increase of production cost.
On the other hand, a decrease in the amount of ethyl acetate causes an increase in the amount of by-products to lower the yield rate and also causes problems in separation due to premature precipitation of crystalline dye during the reaction or the solution-separating procedure to complicate the numbers of procedures necessary to isolate the dye product. These drawbacks represent serious problems from the standpoint of production efficiency.
Also, chloroform-water systems are disclosed as another two-phase system in JP-A-63-264753 and JP-A-64-51991. Owing to much higher solubilities of azomethine or indoaniline dyes in chloroform than in ethyl acetate, the above-mentioned disadvantages associated with the ethyl acetate-water system are avoided. However, chloroform has high toxicity, and such systems have a question as to their yield rate, as described hereinafter. Therefore, production methods free from the foregoing defects have been desired.